Infrared sensors (hereinafter also referred to as IR sensors) can advantageously be used wherever temperatures or temperature differences must be measured. This is the case, for example, in security technology, where approaching persons are sensed by an IR sensor as a result of their body heat, and on that basis, for example, a light is switched on or an alarm triggered. Modern fever thermometers also usually contain an IR sensor, which quickly and reliably measures body temperature via a measurement in the ear.
The advantage of temperature measurement using IR sensors is that even very high temperatures (above 1000 degrees) can be reliably measured, because such a measurement is performed in non-contact fashion, solely on the basis of radiated heat. EP 1 176 407 A2 discusses one such apparatus for non-contact temperature measurement.
The use of an infrared measuring device is also of interest, for example, in scenarios such as underfloor heating systems, heating pipes, etc., i.e. for the surveying of water pipes that are enclosed in floors, walls, or ceilings. Here it might be desirable, for example, to know where the pipes are located so as not to drill into the pipes, or in order to determine the exact location of the pipes if a water-pipe break has already occurred.
Such instances are at present investigated using IR sensor devices, thermal imaging cameras, or thermal films. With such IR sensor devices of the existing art, the region to be examined is slowly scanned and the local temperature maxima are indicated. Thermal imaging cameras generate, in principle, photographs in the infrared region, which can make slight temperature differences visible. Thermal films are made of a temperature-sensitive material that changes color at higher temperatures; the films are intended to be adhered onto the area to be examined. Such methods, some of which are also direct-imaging methods, are collectively referred to as thermography or thermal image recording.
Both passive and active thermography for nondestructive structural diagnosis is known, for example, from the conference paper “Use of active thermography for the detection of inclusions in building structures and in soil” (“Einsatz aktiver Thermographie zur Detektion von Einschluessen in Baukonstruktionen und im Erdreich”) that was presented in the context of the DGZfP Thermography Colloquium in Stuttgart in 2001.
In passive thermography, only the internal quantity of heat radiated from an object being examined is used for measurement. In active thermography, the object being examined is heated actively, i.e. from outside, prior to the examination, and after the external heat source is switched off, the quantity of heat radiated from the object is then detected.
A disadvantage of these thermography methods may be, for example, that an underfloor heating system to be examined should already have been switched off hours before the measurement and should be started up again only shortly before the measurement, so that the temperature difference between regions with and without pipes is sufficiently large. In addition, active thermography requires that not inconsiderable quantities of radiated energy may need to be generated and radiated onto the object.
Because these devices can indicate only temperature distributions, the devices may not be very accurate. External influences such as drafts or room temperature, or also structural features such as, for example, increased thermal diffusion in the floor, can falsify the measured values.
IR sensor devices themselves, and especially thermal imaging cameras, may also be very expensive, since it is believed that they must be very accurate in order to be able to define at least the approximate location of, for example, pipes in concrete.